(3)Suspension Crosslinking Copolymerization method

Suspension polymerization is a method for preparing spherical polymers. Copolymerization is one of the ways to prepare binary or ternary copolymers and improve the properties of polymers. Suspension Crosslinking Copolymerization is a unique polymerization reaction used to prepare ion exchange resin skeleton copolymers.

Suspension polymerization is to suspend the monomer in an insoluble medium, disperse the monomer into liquid beads of appropriate size under appropriate stirring, and polymerize into spheres with uniform particle size. The monomer used to prepare ion exchange resin is generally dispersed into droplets with a diameter of 0.3 ~ 0.8mm by appropriate stirring with non-ionic water as the dispersion medium. The dispersion process of simple mechanical stirring is reversible, and the droplets can aggregate and merge after collision, and finally achieve a dynamic balance to maintain a certain particle size distribution. In order to prevent adhesion due to the collision of balls and beads in the polymerization process, a certain dispersant (or stabilizer) needs to be added to the medium to stabilize the dispersion state of the monomer.

Dispersant has two functions: one is to reduce the surface tension of water and make it easier for monomers to disperse into small liquid beads; Second, it is adsorbed on the surface of liquid beads to protect the liquid from merging and bonding when they touch each other. In suspension Crosslinking Copolymerization, the appropriate dispersant should be selected according to the above principles. Common suspension polymerization dispersants include polyvinyl alcohol, gelatin, starch, methylcellulose and its derivatives, insoluble inorganic substances, such as calcium carbonate, calcium phosphate, talc powder, diatomite, silicate, bentonite, etc. Insoluble inorganic substances mainly isolate the dispersed liquid beads.

In fact, polyvinyl alcohol and gelatin are the most commonly used dispersants in the synthesis of ion exchange resins. Gelatin has the lowest water solubility when its isoelectric point pH = 4.7, which can give full play to its role of protective glue. The disadvantage of gelatin is that the raw materials of different batches and dispersion specifications are not always the same. Polyvinyl alcohol is easier to control in composition and structure than gelatin, and its dispersion ability is stronger than gelatin. It is easier to obtain copolymer with finer particles.

The dispersibility of polyvinyl alcohol mainly depends on its molecular weight and alcoholysis degree. When the molecular weight of polyvinyl alcohol is high, the degree of hydrolysis is low and the dosage is large, the average diameter of the crosslinked copolymer beads is small and the coalescence phenomenon is light. Polyvinyl alcohol, which is generally used as dispersant, has a degree of polymerization of 1000 ~ 2000 and a degree of alcoholysis of 80% ~ 90%. Polyvinyl alcohol with alcoholysis degree of 80% has strong dispersion ability, but poor protection ability. When the alcoholysis degree is 90%, the surface activity is poor, but the protection ability is strong. When synthesizing the parent copolymer of ion exchange resin by suspension copolymerization, it is necessary to disperse the monomer into droplets with appropriate size with appropriate stirring speed and dispersant, and rely on the protective effect of dispersant to prevent the droplets from colliding, merging, aggregation and bonding in the copolymerization process, so as to ensure a high yield of qualified spheres. Therefore, the rational selection and use of dispersant is a very important problem.